The present invention relates to a combustion sensing apparatus having an air fuel ratio sensor incorporating a solid electrolyte such as stabilized zirconia and suited for use in the detection of oxygen concentration in a flame.
In the use of combustion sensor, it is necessary to expose the cathode and anode formed on a solid electrolyte separately to the atmosphere as a reference gas and to the gas to be measured, respectively. In order to make sure of the separation, the solid electrolyte is shaped in a tubular form opened at its both ends of closed only at one end thereof.
The work for forming an anode on the inner surface of such a tubular combustion sensor has to be conducted manually, regardless of whether the tubular body of the sensor is closed at its one end or opened at its both ends. In addition, the contruction of the lead from the electrode is complicated. For these reasons, the cost of production is raised uneconomically and the reliability is deteriorated undesirably.
In the case of the air-fuel ratio sensor, the cathode and anode are produced in the form of porous films, and the catalytic action performed by the porous films largely affects the output characteristics of the sensor. Therefore, a severe control is necessary for the control of the porosity, thickness and other factors of the anode formed on the inner peripheral surface of the cylindrical sensor body. Actually, however, the major portion of the anode cannot be checked visually. This imposes various problems.
The production cost is ruled also by the amount of platinum or the like precious metal alloy used as the material of the electrodes formed on the inner and outer surface of the solid electrolyte tube. Although effort is concentrated to reduce the amount of use of such a precious metal alloy, as a matter of fact, the formation of the electrodes is very difficult so that the alloy is wastefully consumed to form the electrode or leads covering unnecessarily large area, resulting in a raised cost of production.